The present invention relates to a new and improved construction of apparatus for the automatic, uniform drawing-in or infeed of elastomeric materials to worm extruders and to a method of operating such worm extruders.
During the feeding or charging of extruders with a material mixture, the most different types of techniques have been used, and specifically ever since such equipment has been employed for fabricating sectional shapes, profiles, hoses or plates, in order to attain a uniform infeed of the material in an automatic fashion and a uniform pressure in the worm or screw.
What is of primary importance is that there is delivered in an absolutely uniform manner to the extruder the material mixture i.e. the material which is to be processed in the extruder, and equally that such material mixture is uniformly drawn into the worm or screw, so that there is accomplished a uniform, pulsation-free ejection at the injection head or injection disk or equivalent structure. An irregular infeed produces an irregular build-up of pressure which prevails from the rear towards the front in the direction of the exit side of the equipment, i.e. leads to pulsations, and thus, to irregular products having mass differences, since, as a result, also the pressure required for ejecting the material through the injection disk into the injection head or nozzle is irregular.
During the incipient stages of extruder technology, and in part this is also true at the present time, processes were employed working with hot fed extruders. Here, the strip-shaped material mixture was infed in a hot condition, in other words already pre-plasticized, from the rolling mill directly into the worm extruder. Each temperature difference emanating from the rolling mill or during the transport to the worm extruder in the material mixture, therefore had a negative effect and caused irregularities during conveying of the material into the threads of the worm or screw. This is so because colder and thus more rigid or harder, less plasticized material is better able to push material located further forwardly in the worm or screw and which is softer and more intensly plasticized owing to the action of friction and temperature. Hence, in the presence of different temperatures there arises an irregular or different forward feeding or pushing action which is exerted by the newly infed material, which, in turn, leads to pulsations which can be transmitted to the injection disk and thus to the outflowing product.
As to the cold filter-extruders which have been available on the market for a number of years and charged with cold and thus, temperature uniform mixture material-strips, it has been possible to eliminate the irregularities or fluctuations in regard to temperature differences, since the strips within the working chamber rapidly assume the constant or at most slight fluctuations in temperature prevailing in such work chamber. A retrofeed or repulsion of the mixture material back to the filling funnel however can not be directly prevented. The automatic constriction at the injection head or nozzle at the end of the worm (exit cross-section is smaller than the cross-section at the inlet) however leads both for hot as well as cold charged extruders always again to a repulsion or retrofeed of the mixture towards and through the infeed or filling funnel. As a result, the fed back conglomerates or lumps and/or the mixture material-strips are suddenly sheared and the worm or screw is partially empty until material can be again engaged. Due to the foregoing there are formed the previously discussed pulsations.
This requires that at this location there be present an operator for the purpose of observing the operation of the equipment, something which is extremely expensive in terms of the operator's salary requirements, and still he or she is incapable of preventing the damming-up of the material. So that the mixture material-strips can again be engaged, i.e. the worm or screw can again draw such in, the operator removes, with considerable effort, a lump or material conglomerate which has been forcefully pushed upwardly into the filling funnel or pushes such with a rod or the like downwardly. Both of these operations are dangerous, but still are utilized in a number of places. In both cases the uniform infeed of fresh material, and thus, a uniform pressing and infeed forwardly towards the injection head or nozzle is at least periodically interrupted, and therefore, also the uniform pressure of the material in the injection head and the outlet nozzle (injection disk). This leads to fluctuations in the dimensions and quality (compression and density of the material) of the outflowing product and results in a corresponding number of rejects.
Now attempts have been made in the rubber industry on a world wide basis to eliminate these negative effects by bringing about changes at the region of the draw-in zone or infeed zone at the extruder. However, up to the present time no satisfactory solutions have been found.
In lieu of strip-like material there has been infed granulate i.e. grains, something which indeed improves the regularity or uniformity of the infeed of the mixture material due to the possibility of exactly dosing the same by the employment of vibrator troughs. Yet, this technique has not been able to eliminate the damming-up of the material. Additionally, the ejection (capacity) was reduced by about 15%. The production of the granulate also is associated with additional costs and its storage and transport is problematic, since, for instance, soft rubber granulate tends to easily bond to one another. The inclusion of parting or separating compounds or agents, such as for instance, talcum, by virtue of the large surface of the granulate is associated with the danger of impairing the quality of the mixtures.
It is here mentioned that the processing of non-elastomeric plastics is much simpler, since, on the one hand, the granulate does not tend to adhesively bond, and, on the other hand, there can not practically occur any damming-up since thermoplastic material due to the heating-up thereof and the thus occuring plastification as it passes through the worm or screw threads, becomes so soft that there does not prevail within the material sufficient force in order to push such towards the rear. The material therefore simply escapes rapidly towards the front out of the injection disk and therefore does not produce any damming-up. Granulate or granular material for feeding extruders in the form of elastomeric mixtures, especially rubber mixtures, are thus predominantly still only used during cable fabrication since for this purpose there can be employed for the most part markedly filled, stiff mixtures which owing to this stiffness or rigidity possess a larger pressure force than material which has already been somewhat plasticized at the front threads of the worm, and thus, overcome the damming-up force (the same as for plastics). Additionally, the production capacity of extruders employed for cable fabrication is much smaller than for rubber production, such as sectional shapes or profiles, hoses or plates (approximately 40% in contrast to 85%) so that a loss in capacity and a smaller ejection quantity due to the use of granulate is not disturbing.
A widely attempted solution for eliminating the problem of the damming-up of the material resorts to the technique of arranging at the inlet of the material into the extruder a so-called supply or infeed roll which comprises a roll driven in opposite rotational sense to the rotational sense of the worm or screw. While this procedure allows for an improvement as concerns the drawing-in of the mixture material-strips, still here also there can not be eliminated a damming-up and escape of mixture material past the exit.
In the case of large extruders working with about 150 mm worm diameter there also are employed so-called stoppers. A plunger presses the material into the filling funnel. In this way it is possible to push back the damming-up material into the worm threads. Still, with this technique, there remains the pulsating problem which should be avoided, since the stopper or plunger must always again re-engage with the material and thus the applied pressure is not continuous. Also, it is impossible by means of the stopper or plunger to automatically regulate the quantity of material which should be taken-up by the worm. The stopper devices furthermore are constructionally complicated and extremely expensive.
In those instances where processing is accomplished with strip-shaped material, attempts have also been made to work with a technique where there is accomplished a change in the cross-section of the mixture material-strips which are to be infed. This requires a large expenditure and after a certain amount of time nonetheless still does not avoid the dam-up effect.